1. Field of the Invention
This invention relates to a reading signal processing circuit in an optical information reproducing apparatus for optically reading the information out of a recording medium.
2. Related Art Statement
Recently there is practiced an optical information recording and reproducing apparatus wherein, by condensing and radiating a laser light on a recording medium instead of using a magnetic head, information can be optically written in or recorded information can be read out.
In such optical information recording and reproducing apparatus (which may be abbreviated merely as a recording and reproducing apparatus in some case hereinafter), by condensing a laser light, the information can be recorded or reproduced at a density far higher than in the case of a magnetic head.
In the above mentioned recording and reproducing apparatus, there is a position recording or a length recording in the recording system in the recording medium. For example, in the position recording, the information is recorded by making the position of a recording unit (pit) significant. In such case, generally the distance from the center of a pit to the center of the next pit is made significant and this distance corresponds to the pit length in the length record.
The reading signal processing circuit of the position recording system is formed to detect the center of the pit. The size of the formed pit varies with the dispersion of the sensitivity of each recording medium, the dispersion of the sensitivity of the recording medium by the temperature (variation) and the fluctuation of the reading power of the light source. Even if such variation occurs, the center position of the pit will not vary. That is why the position recording system is suitable to obtain a normal reproduced signal.
FIG. 1 shows a block diagram of a reading signal processing circuit in a reading signal processing system of a related art. FIG. 2 is a waveform diagram showing the operation of the reading signal processing circuit shown in FIG. 1.
A reading signal processing circuit 30 shown in FIG. 1 is to bivalue a reading signal detected by a photodetector within an optical head. As shown in FIG. 1, a differentiating circuit 31 inputs and differentiates the reading signal from the photodetector shown in FIG. 2(a). The peak of the reading signal has been converted to a zero crossing point in the differentiated signal as shown in FIG. 2(b). Comparators 32 and 33 have respectively different comparator levels. In the comparator 32, when the input signal exceeds the comparator level, the output signal will be on a high level. In the comparator 33, when the input signal is below the comparator level, the output signal will be on a high level. One comparator 32 is to detect whether there is a pit or not and sets a comparator level Vh to be higher than the zero crossing point of the differentiated reading signal. In case the above mentioned reading signal exceeds the comparator level Vh, this comparator 32 will judge it to be of a pit and will output a gate signal on a high level as shown in FIG. 2(c). As synchronized with this gate signal, a latch signal shown in FIG. 2(d) will be generated within a bivalued signal generating logical circuit 34.
On the other hand, the other comparator 33 inputs a comparator level Vz inverting the output with the zero crossing of the reading signal from the photodetector. The comparator 33 outputs a zero crossing signal shown in FIG. 2(e) to the bivalued signal generating logical circuit 34. When the above mentioned latch signal is on a high level, the bivalued signal generating logical circuit 34 will output a bivalued signal of a required fixed pulse width shown in FIG. 2(f) at the rising edge of the zero crossing signal and then will clear the latch signal. Thus, in response to the pit on the recording medium, the reading signal processing circuit 30 sequentially outputs a bivalued signal but, in some case, noises will be generated in the reading signal as shown in FIG. 2(a). for example, by the flaws and dirts on the recording medium. As shown in FIGS. 2(c) and (e), when the level of the noise is large enough to exceed the comparator level Vh, and a gate signal will be qenerated. Therefore, the bivalued signal generating logical circuit 34 will output a wrong (though there is no pit) bivalued signal. By the way, in case the noise level does not exceed the comparator level Vh, even if a zero crossing signal by the noise is output, the bivalued signal generating circuit 34 will not output a bivalued signal.
However, if the noise components of the input reading signal are few and the noise level is low, the reading signal processing circuit of the above described related art will not be a problem but, in the actual reading very many noises will occur and the signal mixed with noises will have to be processed. There are such various causes of noises as, for example, dust and dirt or flaws on the recording medium, noises generated within such apparatus as a photodetector and noises from a power source line. Thus, in the reproducing apparatus, a noise level larger than the comparator level is likely to be generated and therefore, in the circuit of the above mentioned related art many reading errors by noises may be generated.
Now, as one of the countermeasures against such defects, there is a method wherein a high order filter is put into the input of the circuit but, in case the frequency component of the noise is within the signal band of the filter, it will not be effective. Even in the region where it is effective, the time delay of the signal by the filter will newly become a problem and the circuit scale and cost will both become large.
Also, as another countermeasure, it is considered to make the differential constant of the above mentioned differentiating circuit larger and to make the size of the noise component for the signal component relatively small. However, in the above mentioned conventional reading signal processing circuit, the peak of the reading signal will not be converted to a zero crossing point and a bivalued signal will not be generated in the center of the pit. Therefore, the bivalued signal will be influenced by the size of the pit on the recording medium and the jitters will become large. That is to say, a reading error will be generated.
Thus, in the conventional reading signal processing circuit, a reading error by a noise will be generated and, even if the differential constant is made larger to prevent the reading error, jitters will become larger and a reading error will be generated.